This invention relates to a method of operating an automatic frequency control (AFC) system comprising frequency-measuring means and controllable-frequency signal generating means so as to generate with the signal generating means an output signal whose frequency is dependent on that of an input signal, derived from a first source, for the frequency measuring means. The invention further relates to such an AFC system.
AFC systems are used in various radio receiver and receiver/transmitter arrangements. In some cases, it is desirable for the system to have a very rapid response so that the frequency of the output signal generated by the system is accurately related to that of the input signal within as short an interval of time from the reception of the input signal as possible. A known system described in U.S. Pat. No. 3,761,835 uses an iterative process in which a number representative of the actual frequency of the signal generated by a voltage-controlled oscillator is compared at regular intervals with a number representative of the required frequency (the frequency of the input signal) and the voltage used to control the frequency of the oscillator is changed in an attempt to progressively reduce the difference between the frequencies. However, the amount by which the control voltage should be changed so as to approximately equate the frequencies will generally in practice show a considerable variation over the operating frequency range of the system. Consequently, while the accuracy with which the frequencies are eventually related (in this case equated) with such a system may be satisfactory, the time taken to do so may be very long, and may be unacceptably long even when the number of iterations is comparatively small. An alternative approach which enables a quicker response to be obtained is simply to measure the frequency of the input signal and to control the frequency of the oscillator with an appropriate voltage according to a specified/frequency control voltage transfer characteristic of the oscillator. However, not only may this necessitate the use of particularly expensive equipment, but it has been found that even then the matching of the frequencies is not reliably accurate since the characteristics of the system tend to vary, for example with age and temperature.